1. Field of the Invention
The present invention relates to an optical tomographic imaging equipment adapted to image tomographic images of the subject, specifically a biological sample on a non-invasive basis, and more particularly, to an optical tomographic imaging equipment in which a beam of light is projected to the subject and the light scattered and reflected within the subject is received so that a light-receiving signal, which carries a tomographic image of the subject, can be derived.
2. Description of the Related Art
Hitherto, for observing a tomographic image of the subject, specifically a biological sample, various types of equipment have been known which are based on various principles.
For example, there has been known an ultrasonograph in which an ultrasonic wave is applied to the subject and the ultrasonic echoes reflected by tissues in the subject are received so as to derive an ultrasound-receiving signal. In such an ultrasonograph, a tomographic image of the subject is displayed on the basis of the thus derived ultrasound-receiving signal. This type of ultrasonograph is put to practical use for diagnoses of diseases of, for example, the heart and the abdomen. However, the resolution of the tomographic image is of the order of several hundreds of .mu. meters in terms of the actual size of the subject. The order of several hundreds of .mu. meters is short not less than one figure (digit) in a degree of a lack of resolution to observe in detail a tissue system of a biological sample.
Further, as a device for irradiating beams of light into the subject, there is known a confocal laser scanning ophthalmoscope (hereinafter referred to as "CLSO"). The CLSO adopts a scheme in which a laser beam sequentially scans on a predetermined depth within the subject on a two dimensional basis, or on a three dimensional basis including a direction of the depth, and rays of light reflected from the respective focal points are separately extracted through a pinhole from rays of light reflected from the locations of the depth other than the focal points, so that the extracted rays of light are received. According to this CLSO, it is possible to obtain a two dimensional tomographic image in a short time, such as several tens of m seconds, which image is provided with the resolution as to the horizontal direction (Y direction) of the order of 10.mu. meters. However, the resolution as to the vertical or depth direction (Z direction) is simply of the order of several hundreds of .mu. meters. This order is short not less than one figure (digit) in a degree of a lack of resolution to observe in detail a tissue system of a biological sample.
In view of the foregoing, there has been proposed equipment (refer to PCT/ US92/03536) which is capable of deriving a tomographic image with the resolution of the order of several .mu. meters with respect to both the Z direction (depth direction) and the Y direction (horizontal direction).
FIG. 26 is a view useful for understanding the above referenced equipment.
Light emanating from a light source emitting light with a short coherence length, for example, a SLD (Super Luminescent Diode) 111, is transmitted through an optical fiber 112 and divided by a fiber coupler 113 into a first light wave (object light) and a second light wave which are transmitted through optical fibers 114 and 115 via an objective lens system 117 and a reference lens system 118 to the subject 119 and a reference mirror 120, respectively. The reference mirror 120 is moving in the Z direction (direction of an optical axis of the light beam), while the system is operative.
According to the system shown in FIG. 26, there is provided a PZT (Piezo-electric Transducer) at the object light side to conduct a frequency shift of the object light. The PZT is used so as to obtain at a photo-diode 122, which will be described later, a signal having a frequency suitable for the photo-diode 122 or a signal processing system including the photo-diode 122. However, the PZT is not always needed in view of a principle of the measurement of the system. Thus, an explanation of the operation of the PZT 116 will be omitted and a system having no PZT 116 will be described hereinafter.
The beam of light irradiated onto the subject 119 travels inside of the subject 119, and will be reflected by various points on a traveling path of the beams within the subject. The reflected light is introduced via the objective lens system 117 to the optical fiber 114, and further transmitted via the fiber coupler 113 through an optical fiber 121 to a photo-detector, for example, the photo-diode 122.
Likely, the light reflected from the reference mirror 120 is introduced via the reference optical system 118 to the optical fiber 115, and further transmitted via the fiber coupler 113 through an optical fiber 121 to the photo-diode 122.
FIG. 27 is a view illustrating a signal derived from the photo-diode 122.
The axis of abscissa of the figure corresponds to the location in the Z direction of the reference mirror 120, and also represents a time basis since the reference mirror 120 is moving at constant speed in the Z direction, while the system is operative. The axis of the ordinates of the figure represents an amplitude of a light-receiving signal of the photo-diode 122. A dashed line is representative of the envelope of the light-receiving signal. The signal shown in FIG. 27 will be obtained on the assumption that the reflection of light occurs by only a point within the subject and, in addition, the reflected light and the reference light, which are transmitted to the photo-diode 122, are even in intensity of light.
When the reference mirror 120 is moved continuously at a constant speed in the Z direction, the reference light reflected by the reference mirror 120 is modulated to light of which a frequency transitions by the corresponding Doppler frequency in comparison with the incident light to the reference mirror 120. Consequently, there will occur an interference between the reflected light and the reference light on the photo-diode 122, and thus there will be observed a signal having a frequency given with a difference between the frequency of the reflected light and the frequency of the reference light.
By the way, assuming that the light emitted from the SLD 111 is short in the coherence length and the reflection occurs at only a certain single point of the subject 119, there is observed a burst wave, as shown in FIG. 27, which appears with the origin 0 in the center over only the width in the Z direction (or time interval) corresponding to the coherence length of light emitted from the SLD 111, where an optical path (optical distance), wherein light emitted from the SLD 111 is projected in the form of the object light onto the subject and reflected by a certain point of the subject, and reaches the photo-diode 122, is completely the same as an optical path, wherein light emitted from the SLD 111 is reflected by the reference mirror 120, and reaches in the form of the reference light the photo-diode 122. Actually, reflections occur on various points along the light path of the beam of light travelling inside of the subject 119. Therefore, when the reference mirror 120 is moved in the Z direction, it is possible to derive signals wherein information messages as to the reflection light inside of the subject 119 are sequentially extracted in compliance with the position of the reference mirror 120 in the Z direction at the respective time points during the movement of the reference mirror 120. The full-width-half-maximum (FWHM) of the burst wave in the Z direction in case of reflection from a single point, as shown in FIG. 27, is given with about 10 .mu. meters or less. Thus the resolution of 10.mu. meters or less is obtained with respect to the Z direction (depth direction).
On the other hand, the resolution as to the Y direction (horizontal direction) depends on a degree of reduction of the object beam of light on the reflecting points within the subject 119. In order to obtain the resolution of the order of 10.mu. meters also with regard to the Y direction (horizontal direction), it is necessary to reduce a diameter of the beam at the respective reflecting points to the order of 10.mu. meters. Hence, the objective lens system 117 is equipped with a focusing system. The focusing system is moved in synchronization with the movement of the reference mirror 120 in the the Z direction in such a manner that the object light is converged at the reflecting point within the subject 119 corresponding to the associated location of the reference mirror 120 in the Z direction, so that the focal point of the objective lens system 117 is moved in the Z direction.
In this manner, when the reference mirror 120 is moved in the Z direction, while the focal point of the objective lens system 117 is adjusted, it is possible to derive a line of signal of a tomographic image along a piece of beam of light extending inside of the subject 119. This line is referred to as a scanning line.
The light-receiving signal derived from the photo-diode 122 is supplied to a detector circuit 123 to be subjected to detection in which a signal corresponding to the envelope shown in FIG. 27 is extracted. An A/D converter 124 converts the detected signal from the detector circuit 123 into a digital signal. An output of the A/D converter 124 is applied to a computer 125. Thus, the computer 125 receives image data representative of a tomographic image along a piece of the scanning line.
The objective lens system 117 is movable in the Y direction (scanning direction). When the above-mentioned scanning is repeatedly performed while the objective lens system 117 is moved, the computer 125 receives image data representative of a tomographic image along a two dimensional section consisting of a component as to a depth direction (Z direction) of the subject 119 and a component as to a movement direction (Y direction) of the objective lens system 117. The computer 125 practices a predetermined image processing for the received image data as the occasion demands. Thereafter, a tomographic image involved in the processed image data is displayed on a display unit (not illustrated), or a hard copy of the tomographic image is produced by a hard-copy apparatus.
While the equipment or system shown in FIG. 26 adopts the optical fibers 112, 114, 115 and 121, it should be noted that each of these elements is exemplarily shown as means for transferring light and it is not always needed to use the optical fibers, in view of a principle of formation of the tomographic image which has been explained above referring to FIG. 26.
Incidentally, the above-mentioned proposal includes a system in which a frequency of light source is modulated, instead of a mechanical scanning in the depth direction (Z direction). According to such a system, however, it takes much time for signal processing after light is received by the photo-diode 122, and further it is difficult to converge a light beam on various depth positions. Thus, the system is of a fixed focus. This causes a degradation of the resolution of areas out of the fixed focus point with respect to the lateral direction (Y direction). Consequently, it is difficult to obtain a high resolution of a tomographic image as a whole.
As apparent from the above description, according to the conventional system as explained referring to FIGS. 26 and 27, it is possible to obtain a tomographic image of the subject with the resolution of the order of 10.mu. meters with regard to both the depth direction (Z direction) and the horizontal direction (Y direction).
However, in general, it is difficult to maintain a fine beam within a scattering medium covering a wide field with respect to the depth direction. While the enlargement of the aperture makes it possible to provide a fine beam at the focus point, this permits the beam to rapidly spread. Thus, according to the fixed focus, it is difficult to obtain an even fine beam extending over a wide field. Further, if the aperture is enlarged, it is difficult to attain a high resolution only through simply switching a plurality of transmitter and receiver units. Thus, a mechanical positioning apparatus is needed. There is a technology such that beams are swung at a high speed with a rotary mirror, a galvano-mirror or the like. However, this technology also needs a mechanical positioning apparatus to attain a variable focus in a depth direction.
Hence, it is necessary that as shown in FIG. 26 there are provided a focusing system which is an extremely large optical system in comparison with the reference mirror 120, and in addition the objective lens system 117 having a large aperture, and these systems are moved in synchronism with the movement of the reference mirror 120. It is difficult to move those systems at high speed, and thus it takes a lot of time to derive a light-receiving signal along a scanning line. Further, with respect to the objective lens system 117, it is necessary to move this system also in a Y direction. As a result, it would take a long time to make up a piece of two-dimensional tomographic imaging data. While the taking of much time to issue the tomographic image is a problem itself, it causes the following drawbacks. For example, in a case where the subject is a biological sample in vivo, the subject will significantly move during the time required for obtaining the tomographic image. Thus, it is difficult to obtain a proper tomographic image at a certain point of time.
Further, the objective lens system 117 is constituted of, for example, a lens group equipped with a focusing system. In addition, it is necessary to provide a mechanism for moving the objective lens system 117 also in the Y-direction. Consequently, if it is intended to implement the apparatus shown in FIG. 26, the apparatus involves such a problem that the mechanism of the apparatus becomes complicated and the apparatus is of a large size. These drawbacks result in reduced reliability, and the apparatus becomes expensive.